In the prior art, faults in dual homed configurations of nodes in communication networks are usually overcome by changing a path in the configuration, accompanied by flushing address tables in the nodes. Let us discuss a configuration shown in FIG. 1, where two peer nodes of a dual homed configuration residing in a first network (say, gateways A, B of an aggregation network or a transport network)—are connected to one common node F—let it be called a source node. In the dual homed configuration, the two gateways A, B are respectively connected to two nodes C, D of a second network (say, of an access network). Such a configuration may serve for unilateral protection (or Y-projection) of a specific node—say, node C in the second network, which is adapted to receive traffic from any of the gateways A, B. In case a fault occurs in one of the gateways of such a configuration (say, in the main/primary node A being currently in communication with the common source node F), the dual homed structure must react to the change of status of the gateways as follows: the traffic must be established via another (secondary) node B of the configuration, and to do so a so-called MAC flush operation must be performed in the source node F (“z” marks a MAC table of e node F).
The problem of changing status of the peer nodes is often in synchronizing the further operation of the nodes. For example, the problem becomes actual in case when the dual homed configuration returns to normal upon some fault of the primary gateway has been fixed. In that case, communication from the common source node F should be rerouted back (to pass via the primary gateway again), and it means that the flush operation in the node F has just been performed due to the fault, seems to become necessary again. Such a flush operation is indeed performed in all prior art solutions. Usually, OAM signaling messages are utilized for such purposes. However, those skilled in the art know that any MAC flush operation not only consumes time and resources of the system, but also leads to missing some data due to the rerouting. The effect of missing data is sometimes called “black holing” and takes place, for example, before and during the flushing of MAC addresses due to change of status of the dual homing nodes and paths.
Various prior art solutions are known in the field, for example:
US2009168647A describes inter-working an Ethernet Ring network with an Ethernet network with traffic engineered trunks (PBT network) and enables traffic engineered trunks to be dual homed to the Ethernet ring network to allow protection switching between active and backup trunk paths in the PBT network. In one embodiment, the active path will terminate at a first bridge node on the Ethernet ring network and the backup path will terminate at a second bridge node on the Ethernet ring network. Trunk state information is exchanged between the bridge nodes to enable the bridge nodes to determine which of the active and backup paths should be used to forward data on the trunk. Upon a change in trunk state, a flush message is transmitted on the Ethernet ring network to enable the nodes on the Ethernet ring network to relearn the path to the new responsible bridge node.
CN101330424A discloses a method for handling a service failure of a virtual private network and relates to the technical field of network communications. The method comprises the following steps: a second provider edge device (PE) sends a Hush message to a switch when a failure occurs between a first client edge (CE) device and a second CE device, wherein the second PE device turns into a forwarding state; eliminating a medium access control (MAC) address list of the switch according to the Flush message; eliminating a MAC address list of a third PE device. The invention further discloses a VPN failure handling system, which comprises the switch, a first PE device, the second PE device and the third PE device. The method solves the problem that a CE dual-homing network switch interface of a virtual private LAN service (VPLS) network can not update in time, so as to improve the reliability of the VPLS network.
Internet Draft “Flushing-free MAC address Operation in VPLS with Redundancy” (draft-jiang-12vpn-vpls-mac-operation-ol.txt) by Y. L. Jiang et al, describes a case when that Peer Elements PE1 and PE2 of a dual homing configuration may switch some forwarding items affected by a topology change, rather then flushing them (i.e., to perform so-called MAC address moving). For example, PE may actively switch the MAC addresses in its MAC address table which are associated with the failed access link. After detection of the failure of the primary access link, PE1 sends out a “MAC address switching” message to all its peers in the VPLS, with addresses of PE1 and PE2 in the message and with a MAC list of the MAC addresses associated with the access link to be switched, or with a null MAC list when all access links attached to PE1 in the same VPLS are to be switched. PE1 can actively switch the MAC addresses in its MAC table, which are associated with the failed access link.
In any of the above-mentioned documents the operation of switching from one peer (GW) of the dual homed structure to another (including the case of returning to the primary peer) is accompanied either with the flushing, or with the MAC switching (performed by signaling), both of which comprise complex and traffic-affecting actions.